Polyimides are useful as components which require excellent thermal, electrical and/or mechanical properties. For general discussion of polyimides preparation, characterization and applications see Polyimides, Synthesis, Characterization and Applications, K. L. Mittal, ed Plenum, NY 1984.
Polyimides based on pyromellitic dianhydride and various organic diamines are disclosed in U.S. Pat. No. 4,485,140 to Gannett et al (E. I. DuPont de Nemours and Co.).
Polyimides based on diamines such as 2,2'-di-(p-aminophenyloxy)diphenyl and various dianhydrides are disclosed in U.S. Pat. No. 4,239,880 to Darms (Ciba-Geigy Corp.).
Harris et al. in U.S. patent application Ser. No. 07/315,327, has disclosed the preparation of soluble polyimides based on polyphenylated dianhydrides. The polyimides taught in this reference are typically rod like polyimides and possess little if any thermoplastic properties.
In U.S. Pat. No. 4,271,288, 4,239,694 and 4,421,929, Woo (DOW Chemical Co.) teaches the use of certain tetracarboxylic acids as condensation monomers with diamines including oxyalkylene and alkylenedioxy diamines. However, none of these diamines have geminal alkyl groups beta to the carbon atom bearing the aromatic amino end group.
Numerous patents deal with the manufacture of composites utilizing carbon fibers or other similar fibrous reinforcing agent with polyimides. In U.S. Pat. No. 4,85 1,280, Gupta teaches the use of carbon fiber reinforced polyimide composites for fabricating tools. Gupta teaches the use of a different class of polyimides than those employed in the present invention.
In U.S. Pat. No. 4,395,514, Edelman teaches a process for the preparation of polyimide composites including carbon fiber reinforced polyimide composites. The main thrust of the Edelman patent is the use of a class of cyclic peroxymetal catalysts. Edelman teaches the use of divalent aryl radicals.
Recently, with advances in electric circuitry, copper-clad laminates have been used in new ways and required to have superior characteristics. Particularly, an increase in wiring density has been required, leading to lamination of wiring boards and also to size reduction of the through holes. In these circumstances, there is a demand for copper-clad laminates, which are processible under mild conditions, and less subject to smear generation during drilling. Meanwhile, there are also demands for productivity improvement and cost reduction dictating more and more stringent processing conditions in actually mounting wiring boards, particularly in connection with the hot air leveler or reflow soldering. Key requirements are superior heat resistance and moisture resistance of the copper-clad laminates as substrates than heretofore obtainable.
To meet these demands, there is a trend for utilizing additionally hardened polyimide resins in lieu of epoxy resins, which have been finding extensive applications for copper-clad laminates. It is well known in the art that polyimide resins, when utilized for copper-clad lamination, are advantageous in that substantially no smear is generated during drill processing and that their heat resistance during processing is improved.
However, the prior art hardened polyimide resins have posed the following problems. For example, while Kerimid.TM. resins, which are primarily composed of a combination of bismaleimide and 4,4'-diaminodiphenyl methane, have excellent lamination characteristics, 4,4'-diaminodiphenyl methane used in the synthesis is highly reactive, thus posing a shelf-life problem. The varnish and prepreg using it can be used for only a short period of time. Additionally, 4,4'-diaminodiphenyl methane is toxic to the human body. Kerimid processing requires heating at high temperatures for long periods of time, which is a significant disadvantage. Further, those resins which are obtainable from bismaleimides and diamines, are inferior in their moisture resistance. Therefore, preservation of the obtained laminates has traditionally required detailed attention to moisture absorption.
In order to solve the above problems inherent in polyimide, many improvements have been proposed, particularly various polyester imide resins, such as those described in U.S. Pat. Nos. 4,757,118, 4,362,861, and 3,852,246, and Japanese Patent Application Laid-Open No. 1-123819. Those resins which are obtainable by reacting N,N'-bisimide of unsaturated dicarboxylic acids and aminoethyl benzoate, although suitable for laminates, are inferior in their solubility in low-boiling solvents and pose problems in their coating on glass cloth or fiber or the like when producing prepregs. Further, their solutions must be preserved with care.
None of the above composite patents teach or insinuate that the polyimides or polyimide composites of the present invention would have the unusual properties necessary.